1. Field of the Invention
This invention relates generally to xerographic high-speed printing, or xeroprinting, and specifically to a novel xeroprinting master sheet and drum assembly for use in a xeroprinting system.
2. Description of the Prior Art
Xeroprinting, as described by R. M. Schaffert in Electrophotography (1980), at p. 209, is an electrostatic printing process designed to print many copies of a subject, document or page from a master plate. The principle of operation, using a master plate formed as a master cylinder, is shown in FIG. 1. The master cylinder typically consists of a conductive substrate, such as a metal sheet or cylinder, upon which is imprinted an image in the form of an insulating coating by conventional photomechanical methods. The image areas consist of insulating material and the background areas are the surface of the bare substrate.
The xeroprinting master is mounted in a xeroprinting engine so as to be rotated past a series of xeroprinter subassemblies, or stations. When the master is electrically charged by passing it under a corona discharge wire, the charges deposited on an insulated image area remain on the image surface and those deposited on the bare metal areas are conducted away to ground. Thus an electrostatic image corresponding to the insulated areas is formed. Thereafter, the making of a print follows a sequence of steps well-known in xerography; e.g., the image is developed with a pigmented powder (toner) and transferred to a receiver, typically paper.
U.S. Pat. No. 2,576,047, to R. M. Schaffert, provides another description of the xeroprinting process. Initially, the master plate is formed as a drum having a grounded, conductive substrate. A developed, insulating primary toner image is transferred and fused to the electrically conductive plate. The plate is charged and whereas the untoned background areas retain no charge, a charge remains on the plate surface in the toned portions. The electrostatic charge pattern, or latent image, can then be developed with a secondary toner and transferred to a suitable receiver for fusing thereto. The drum is then cleaned, and the recharge, and development, and image transfer process is repeated to successive receivers.
A method is disclosed for electrostatic printing in U.S. Pat. Nos. 3,954,463, and 3,615,128, issued to Bhagat. The method includes charging a photosensitive plate and exposing it to an optical pattern to form a latent image; placing the plate on a conveyor; developing the latent image and affixing (fusing) the developed image to the plate to form a xeroprinting master; the aforementioned xerographic steps are utilized as a continuous printing operation to charge and develop the plate, as described by Schaffert. The unfused secondary toner then can be transferred to a copy sheet or web thereby creating a copy of the master.
In some disclosures in the art of contact printing, an original sheet or master has been illuminated from within the drum.
U.S. Pat. No. 3,642,368, issued to Moss, discloses a cylindrical exposure device wherein a transparent original or master sheet is attached to the outside of a drum which has internally rotating slit light sources. A light sensitized plate, attached to the exterior of the drum, is exposed and then removed to be developed in a separate apparatus.
U.S. Pat. No. 4,167,326, issued to Payne, discloses a contact photocopying process and apparatus wherein no master is used. A drum of translucent material has an internal light source which illuminates the rear of a reversed document or original. The illuminated print simultaneously contacts a charged sensitized paper at the nip between the drum exterior and another drum. The latent (correct) image on the sensitized paper is developed and the paper is removed.
U.S. Pat. No. 3,893,762, issued to Inagaki, discloses a master paper winding drum which includes a drum having a resilient layer and a flexible electrically conductive outer layer.
However, the objective in xeroprinting is the production of a relatively large number of copies of a single original. This objective is best accomplished by the use of a master generated from the original, rather than by pursuing the direct reproduction of the original. Some xeroprinting masters that have been created according to the prior art require substantial manufacturing processes and expense. For example, some xeroprinting masters are constructed of materials that retain a master image by forming image-wise patterns in a photoconductor film composition after exposure to a high level ultraviolet-light image. Such a master and the related exposure apparatus system may increase the cost and complexity to the system.
During development of the xeroprinting master, the charged secondary toner particles are electrostatically attracted to oppositely-charged image areas (which are made up of fused primary toner particles). Since there is no charge on the background areas, no attraction of toner particles to the background areas would be expected. In practice, however, Van der Waals forces cause some toner particles to mechanically adhere to the uncharged background areas, and the unwanted residual toner eventually transfers to the receiver. This tends to create an undesired background coloration in the developed image. The master must therefore subjected to a cleaning process after each secondary toner transfer to remove residual toner remaining on background areas of the surface. This is especially important with respect to a xeroprinting master, as compared to xerographic copiers in general, since a build-up of background toner will occur rapidly in a machine operating at high speed. Proper cleaning of the master is thereby crucial to good xeroprinting.
Consequently, the cleaning methods and apparatus in the prior art must at times be so thorough that the master surface is subjected to abrasion or degradation. Accordingly, practitioners of the art have resorted to complicated means for protecting the master. For example, by coating the conductive substrate of a xeroprinting master with a thin layer or coating of a lubricous resin, there is a reduced tendency of secondary toner particles to be retained in background areas during development. The cleaning of residual toner particles from the background areas is then facilitated due to the coating overlying the substrate. However, the apparatus and process for establishing such a coating increases the expense and complexity of those masters. Further, the coating on such a master is itself subject to depletion with concomitant secondary toner adhesion.
Some photosensitive image receptors used in xerography have been known to include protective layers that are transparent and insulative.
U.S. Pat. No. 4,664,500, issued to Kohler et al., discloses a method and apparatus for transferring a latent image from a photosensitive image receptor to a dielectric or insulating image receptor. The photosensitive image receptor comprises a conductive base, a photoconductive layer, and a transparent protective layer.
U.S. Pat. No. 3,784,398, issued to Metcalfe et al., discloses a method and means for transferring recorded signals and latent electrostatic images wherein an insulating medium is placed over the photoconductor surface.
However, these and other efforts evident in the prior art do not fully provide a simplified, abrasion-resistant xeroprinting master for repetitive, extended use in a high-speed xeroprinting engine. There remains the task, at high speed, to clean such a master thoroughly, and yet the primary (fused) toner pattern must not be removed or damaged. Removing the master from the xeroprinter (for cleaning) is also contrary to the basic premise of a high-speed, efficient xeroprinting operation. A master that is simple, efficient, inexpensive, and durable, and which may remain on the xeroprinting engine for extended periods, is therefore needed to realize the advantages of the xeroprinting process that was envisioned by Schaffert.
There is also a need for a xeroprinting master that may be formed from conventional materials into a film of simple structure. A large number of blank masters could be fabricated and subsequently provided with a master image by processing it in a master-making reprographic apparatus using conventional xerographic techniques.